The present invention pertains to laminates which absorb or reflect electromagnetic radiation in a predictably variable manner and can thus be used to control the heat absorbed or directed away from an underlying structure. Laminates of the present invention comprise at least one thermochromic layer and at least one reflective layer and vary predictably with response to their properties of heat absorption and reflection. They are thus thermoabsorptive-reflective dynamic laminates useful, for example, in effecting methods for controlling the absorption or reflection of radiant or heat energy into or away from of structures that underlie the laminates.
It is known that thermochromic compositions change color in response to temperature fluctuations. In their simplest form, thermochromic compositions are special combinations of chemical compounds and organic materials that exhibit color or transparency to light in response to temperature fluctuations. Typically, upon reaching or exceeding a trigger temperature, a thermochromic composition exhibits transparency to thermal or xe2x80x9cUVIxe2x80x9d radiation and, upon falling below a trigger temperature, the thermochromic material will exhibit color, and, correspondingly, opacity to radiant energy. The thermal or xe2x80x9cUVIxe2x80x9d radiation range is the range of frequencies in the Ultraviolet, Visible and Infrared ranges which produce heat in objects, which absorb those frequencies. The relative transparency/opacity of a thermochromic material is dynamic with respect to the trigger temperature or trigger temperature range of the specific thermochromic material. For example, a thermochromic material having a trigger temperature of precisely 72 degrees Fahrenheit would be opaque to light and thus would absorb thermal radiation in the UVI range at temperatures below 72 degrees Fahrenheit and transparent, or non-absorptive, to UVI light at temperatures above 72 degrees Fahrenheit.
Materials are known that possess the properties of color-exhibiting and color-extinguishing to a transparent state when exposed to temperature change above their trigger temperatures. Materials are known which can go through numerous cycles of changing between color-exhibition and color-extinguishing to transparency. Such materials are described, for example, in U.S. Pat. Nos. 5,919,404 and 5,558,700, which are incorporated herein by reference.
All patents cited herein are incorporated by reference.
It is known that modifying ratios of the compounds used to produce a thermochromic composition can control the trigger temperature and relative sensitivity of coloration/transparency of thermochromic materials. In addition, the maximum density of the color displayed when the thermochromic composition is in the color-exhibiting state can also be controlled to be either translucent (i.e. partially transparent) in varying degrees or to be fully opaque. For example, U.S. Pat. No. 5,585,425 describes a method for creating a thermochromic opaque/transparent composition, laminate member. Other patents relevant to methods for manipulating the properties of thermochromic materials are described in U.S. Pat. Nos. 4,028,118 and 5,919,404.
It is also known that thermochromic compositions can be produced in a range of different colors. For example, U.S. Pat. No. 5,919,404 describes a method for creating reversible thermochromic compositions that exhibit a wide range of traditional colors, while U.S. Pat. No. 5,558,700 describes a method for creating reversible thermochromic compositions that exhibit fluorescent colors. It is likewise known that thermochromic compositions can be laminated to various substrates depending on the desired application, i.e. U.S. Pat. Nos. 5,352,649 pertaining to a thermochromic laminate member, and composition and sheet for producing the same; U.S. Pat. No. 5,688,592 (xe2x80x9cShibahashi ""592xe2x80x9d); and U.S. Pat. No. 5,585,425.
In a more complex form, thermochromic compositions can be produced in the form of microcapsules using conventionally known methods to protect the material from external elements, maintain their functionality and to endow them with desirable properties and characteristics. U.S. Pat. Nos. 4,028,118 and 5,919,404 are good examples of patents that describe known properties of thermochromic compositions.
It is also known that thermochromic materials can be added to thermoplastics, polyvinyl chloride (PVC) or other resins and molded into any shape or design or made into sheets (as described in U.S. Pat. Nos. 4,826,550 and 5,919,404). For example, U.S. Pat. No. 5,798,404 describes a method in which hair curlers are manufactured with a thermochromic thermoplastic elastomer body.
Significant research has gone into increasing the light-fastness of the colors of thermochromic materials as perceived by the human eye, increasing luminosity (brightness) and reducing the fading of colors that may be caused by the cycling of sunlight, and especially by ultraviolet frequencies. The purpose of such research is to develop formulations of thermochromic materials that are more resistant to damage from light, which impinges upon them. The Shibahashi ""592 patent describes an example of a layer of thermochromic material (blue in color) which was covered with a UV filter layer, which is dark yellow in color. When the thermochromic material was in the color developed stage, the additive color rule applies, that is, yellow+blue=green. Therefore the perceived color of the material in its color-developed stage is green and not blue. A solution to this problem presented in the Shibahashi ""592 patent is to combine two layers the first of which is a color reflecting layer comprising particulates of natural mica coated with a reflective metallic luster pigment such as titanium oxide sprinkled on the layer. An additional feature is to provide, over the thermochromic layer, a layer of UV absorber, which filters out UV light. One of the functions of the reflective layer is to reflect some of the light before it hits the UV absorber and the thermochromic layer to thereby present the true color of the underlying layer. The Shibahashi ""592 patent does not, however, recognize the advantages of utilizing the variable transmissivity of thermochromic materials to control the thermal absorption or reflectance of a structure.
Thermochromic laminates of the present invention, and methods for using them, comprise a passive system that automatically varies the reflection or absorption of electromagnetic energy in response to temperature changes. Thus, the present laminates function with respect to predetermined temperature trigger points or ranges without the input of energy except from that of the incident radiation. Laminates of the invention comprise at least two layers, a thermochromic layer having a trigger temperature or a trigger temperature range, and a reflective, or partially reflective layer that is concealed from or exposed to radiant energy incident upon the outer laminate by the change in transmissivity and corresponding color change of the thermochromic layer. The interaction of the at least two layers with respect to the absorption or reflection of radiant or thermal energy can be used to control or modulate the absorption of heat or other energy by an underlying structure. By doing so, the present invention diminishes the need for the use of conventional energy sources such as electricity or natural gas to control the temperature of the underlying structure. Further objectives and advantages will become apparent from a consideration of the following description and attached drawings.
It is an object of the present invention to provide thermochromic laminates that can be utilized to control the heat gain or loss of an underlying structure without the necessity for an external power source.
It is a similar object of the present invention to provide thermochromic laminates that predictably vary their absorption or reflectance of incident radiant energy in response to temperature changes.
It is also an object of the present invention to provide thermochromic laminates that predictably effect color changes in response to changes in temperature. Yet an additional object of the present invention is to provide methods for using thermochromic laminates to regulate the thermal energy absorption or reflection of a structure to thereby control its temperature.
In accordance with this and other objects of the invention, a thermochromic laminate is provided comprising a base layer, the base layer having a structure contacting surface for communicating with an underlying structure such as an airplane hanger, an automobile, a house or other building, and a thermochromic contacting surface for communicating with the thermochromic layer of the laminate. The base layer is both substantially reflective to electromagnetic radiation and substantially conductive to heat. Laminates of the present invention also comprise a thermochromic layer, the thermochromic layer having a base layer contacting surface for communicating with the base layer and an outer surface for communicating with electromagnetic radiation, for example from the sun. The thermochromic layer is of variable transmissivity to the electromagnetic radiation, the extent of the transmissivity being determined by the temperature of the thermochromic layer relative to its trigger point such that a change in the temperature results in a change in the transmissivity of the thermochromic layer and a change in the rate of heat transferred to, or reflected from, the underlying structure.
With laminates of the present invention, an increase in the temperature of the thermochromic layer above the trigger point or zone results in a color change to a more transparent state corresponding to an increase in the transmissivity of the thermochromic layer so that the amount of electromagnetic radiation reflected from the base layer through the thermochromic layer and away from the underlying structure is increased and the amount of heat available for absorption by the base layer is thereby decreased. Similarly, a decrease in the temperature of the thermochromic layer below the trigger point or zone results in a color change to a less transparent, or more opaque or colored state corresponding to decrease in the transmissivity of the thermochromic layer so that the amount of electromagnetic radiation reflected from the base layer through the thermochromic layer and away from the underlying structure is decreased and the amount of heat available for absorption by the base layer is thereby increased. Thus, the thermochromic layer of laminates of the present invention functions to vary predictably the accessibility of the reflective base layer to electromagnetic radiation. Thus, the more accessible the reflective layer becomes, the greater the proportion of the incident radiation that will be reflected away from the structure. The converse is true also.
Laminates of the invention can go through numerous temperature change cycles and still retain their advantageous features. The increase in the transmissivity of a thermochromic laminate of the present invention is reversible when the temperature decreases below a particular temperature trigger zone for the laminate. Similarly, the decrease in transmissivity of a thermochromic laminate is reversible when the temperature increases above the particular temperature trigger zone for the laminate. Typically, an increase or decrease in the transmissivity of the present thermochromic laminates is accompanied by a change in the opacity, color or both, of the thermochromic layer.
The base layer of the present laminate is typically metallic in nature, for example, comprising a metal such as aluminum having the ability to reflect sunlight and conduct heat. In some cases, a metallic coating on the underlying structure, or on an underlying heat-conductive fabric, will be sufficient to provide the reflective and heat-conductive and heat-emissive requirements of the base layer. Base layers of the present invention are not limited to metals, however. Any material that is both reflective and sufficiently heat-conductive to carry out the functions of the base layer can be used to form laminates of the present invention. Mylar is suitable for certain embodiments of the present invention.
In some embodiments, laminates of the present invention are attached to an underlying structure, such as the roof and exterior walls of a building, to the exterior surfaces of a greenhouse, or to the exterior surfaces of a desert shelter. Advantageously, the emissivity of the base layer can be matched to the specific environment in which the underlying structure exists. For example, in some applications, it may be desirable to use a thicker aluminum sheet as a base layer in order to delay the time required for incident radiation to be conducted into the underlying structure.
Laminates of the present invention are particularly useful in applications where the electromagnetic radiation impinging upon them is sunlight. Other forms of radiation, such as from artificial sources such as heat lamps or kiln fires, would also serve to operate the present laminates. An important characteristic of the laminates of the present invention is that they are passive in nature, that is, they require no external energy source other than that of the impinging radiant energy.
Another significant aspect of laminates of the present invention pertains to their temperature trigger zone, that is, a narrow temperature range in which occurs the transition of the thermochromic layer from being highly transmissive to electromagnetic energy such as sunlight, to being highly opaque, or from being highly opaque to being highly transmissive. The present laminates can be made to have a set temperature trigger zone around a specific trigger temperature point. The trigger zone might therefore be several degrees C. or less than one degree C. For example, a laminate might be set to be highly transmissive, that is, transparent or nearly transparent to visible, UV and infrared light at a temperature above 25 degrees C., and colored or opaque at temperature below 22 degrees C. Thus, the temperature trigger zone would be from 22 to 25 degrees C. Within the trigger zone, the degree of transmissivity/opacity/color would vary depending upon the particular laminate. Laminates of the present invention can be made to have wider or narrower temperature trigger zones as desired, and the trigger zones can be set at desired high and low temperature points.
Laminates of the present invention can be formed of materials that can be molded to fit the shapes of surfaces of the underlying structure, in forms sufficiently malleable to conform to irregular surfaces, for example, in a sheet like form sufficiently flexible to be wrapped over or around objects. Laminates of the present invention can also be combined with or incorporated into currently available materials that are used to build or cover underlying structures, for example, as the outside layer of vinyl siding materials. Laminates of the present invention can be made, for example, of a thermochromic layer that comprises at least one electron-donating phenolic compound, and that is enclosed in microcapsules. Moreover, the thermochromic layers of the present laminates can be provided in colors, which are coordinated to the esthetic appearance of the underlying structure, or to the local environment. Preferably, the thermochromic layer is provided in a color that optimizes the amount of thermal energy absorptivity in a given application of the laminate.